Conventional detectors for detecting infra-red radiation by means of a detecting arrangement contained in a Dewar type vessel include a window provided in the wall of the vessel through which the infra-red radiation can be transmitted to the detecting equipment. Generally the radiation to be detected is coupled to the detector by means of an infra-red transmitting fibre which typically has a large numerical aperture. The coupling of large numerical aperture infra-red fibres to a detector presents a number of problems. These include the large cost of infra-red transmitting lenses which are usually necessary and aberration of such lenses. It is also difficult to locate mirror assemblies with large apertures in small spaces. There is also a problem that modal noise which occurs by over-filling detectors when using large diameter fibres. Other problems can result from misaligned detector optics.